The present invention relates to gas turbine engines, especially to a fuel and air mixer for a gas turbine combustor, and more particularly to a gas-liquid mixer which may be used as a mixer of a combustor for the type of gas turbine engine which may be used in power plant applications.
Low NOx emission levels from a turbine engine, of below 10 volume parts per million (ppmv), are becoming important criteria in the selection of turbine engines for power plant applications. The current technology for achieving low NOx emissions may require a fuel/air premixer. Combustors that achieve low NOx emissions without water injection are known as dry-low-emissions (DLE) and offer the prospect of clean emissions combined with high engine efficiency. The technology relies on a high air content in the fuel/air mixture.
In a DLE system, fuel and air are lean-premixed prior to injection into the combustor. No diluent additions, such as water injection are needed for significantly lower combustion temperatures, which minimizes the amount of nitrogen oxide formation. However, two problems have been observed. The first is combustion instability or unstable engine operability which results in decreasing combustion efficiency. The stability of the combustion process rapidly decreases at lean conditions and the combustor may be operating close to its blow-out limit because of the exponential temperature dependence of chemical reactions. This also can lead to local combustion instabilities which change the dynamic behavior of the combustion process, and endangers the mechanical integrity of the entire gas turbine engine. This is because several constraints are imposed on the homogeneity of the fuel/air mixture since leaner than average pockets of mixture may lead to stability problems, and richer than average pockets will lead to unacceptably high NOx emissions. At the same time, a substantial increase in carbon monoxide and unburned hydrocarbon (UHC) emissions as a tracer for combustion efficiency is observed, which is due to the exponential decrease in chemical reaction kinetics at leaner mixtures for a given combustor.
It has been found that a key requirement of a successful DLE catalytic combustion system is the reaction of a perfectly mixed gaseous fuel and air mixture that has less than a 5% variation in fuel/air ratio.
It is also desirable that gaseous and liquid fuels be selectively used for the combustion process under different conditions during engine operation. For example, liquid fuel may be used in a backup system for emergencies while gaseous fuel is used for normal operation.
It is an object of the present invention to provide a fuel and air mixer which is capable of providing a better fuel/air mixture.
It is another object of the present invention to provide a gas and liquid mixer which is capable of providing a fuel/air mixture using both gaseous fuel and liquid fuel.
It is a further object of the present invention to provide a fuel and air mixer which is relatively convenient to manufacture.
In accordance with one aspect of the present invention, a mixer for a gas turbine combustor is provided. The mixer comprises a chamber having a substantially truncated conical shape with an upstream end having a diameter smaller than a diameter of an open downstream end of the chamber. A truncated conical annulus at the downstream end thereof communicates with the chamber at the upstream end thereof. The truncated conical annulus thus has a diameter at the downstream end thereof smaller than a diameter of an upstream thereof. The mixer includes a first fuel injection means disposed in the annulus for injecting fuel into the annulus, and a plurality of upstream air passages communicating with the annulus. The upstream air passages are located upstream of the first fuel injection means for supplying air flow into the annulus to mix with the fuel injected into the annulus, thereby forming a fuel and air mixture. The mixer further includes a plurality of downstream air passages communicating with the chamber. The downstream air passages are located adjacent to the upstream end of the chamber for introducing air flow to further mix in the chamber with the fuel and air mixture.
The fuel injected from the first fuel injection means is mixed with air in the annulus, and the fuel and air mixture flows downstream into the chamber and is further mixed with the air introduced from the downstream air passages. When the air flow from the upstream air passages and the mixture formed in the annulus travel downstream through the annulus, the velocity of fluid flow increases since the cross-sectional area of the annulus decreases from the upstream end to the downstream end. The increased velocity of fluid flow improves the mixing of fuel and air.
It is preferable to provide a central passage communicating with the chamber at a center of the upstream end thereof for supplying air flow into the chamber. The central passage preferably comprises a second fuel injection means adjacent to the bottom of the chamber for injecting fuel therein to mix with air. The second fuel injection means is adapted to operate independently from the first fuel injection means in the annulus so that the second fuel injection means may be used for optional liquid fuel injection while the first fuel injection means is used for gaseous fuel injection.
More specifically, a mixer for a gas turbine combustor according to an embodiment of the present invention, is formed with a body member having a central axis extending between opposed upstream and downstream ends. A central chamber is formed in the body member, including a truncated conical section. The chamber extends inwardly from the downstream end of the body forming an open end thereof, and terminates inside the body member forming a bottom thereof. The bottom has a diameter smaller than a diameter of the open end. A truncated conical annulus is formed in the body member upstream of the chamber. The annulus includes a small end and a large end. The annulus communicates at the small end thereof with the bottom of the chamber. A plurality of upstream air holes extend inwardly from the upstream end of the body member in fluid communication with the annulus and the exterior of the body member, for introducing air flow into the annulus. A plurality of hollow spokes extend radially in the annulus and are disposed in a circumferentially spaced apart relationship. Each of the hollow spokes includes a plurality of first fuel holes for injecting fuel into the annulus to mix with air, thereby forming a fuel and air mixture. A plurality of downstream air holes extend through the body member in fluid communication with the truncated conical section and the outside of the body member for introducing air flow into the chamber to further mix with the fuel and air mixture.
The body member preferably comprises a central passage extending axially from the upstream end thereof to the bottom end of the chamber for supplying air flow into the chamber. The central passage preferably comprises a plurality of second fuel injection holes adjacent to the bottom of the chamber for selectively injecting fuel to mix with air. The upstream and downstream air holes are preferably in angled orientation to create air swirl which further improves the mixing of fuel with air.
For convenience of manufacturing, the body member preferably comprises a base body including the chamber and a truncated conical cavity forming an outer wall of the annulus, and an end body including a plate and a truncated conical central member extending from the plate and oriented perpendicular thereto. The plate forms the upstream end of the body member and the central member forms an inner wall of the annulus when the end body is assembled together with the base body.
The mixer, according to the present invention, improves the mixing of fuel with air to increase the flame stability, especially under lean conditions, and is convenient to manufacture.
Other advantages and features of the present invention will be better understood with reference to a preferred embodiment of the invention described hereinafter.